1. Field of the Invention
The present invention relates to a process for the production of nylon 6 in which a feed stream that contains a major portion of caprolactam and/or nylon 6 prepolymer and a minor portion of 6-aminocapronitrile is fed directly to a multistage reactive distillation column where it is reacted with a contercurrently flowing steam stream.
2. Related Art
Nylon-6 is produced commercially using caprolactam as the starting material. This process typically involves heating caprolactam in a VK tube at atmospheric pressure for 12 to 24 hours. This process produces a nylon-6 product that contains 7 to 10% caprolactam in the product. Excess caprolactam is extracted using an 8 to 12 hour aqueous extraction process. The extracted product is then dried for 8 to 12 hours producing a dry product containing 0.2 to 0.3% caprolactam. While this process has met with commercial success, the purification of the nylon-6 product adds a considerable expense to the process.
U.S. Pat. No. 2,245,129 discloses a process for the production of polyamides by a two step process that first reacts an aminonitrile compound with water in a closed vessel to produce a low molecular weight polyamide product. In a second step, heating the product with venting to remove ammonia and excess water increases the product""s molecular weight. A disadvantage of this process is long reaction times and difficulty achieving desirable molecular weights.
U.S. Pat. No. 6,069,228 discloses a process for the conversion of 6-aminocapronitrile to nylon-6 using a pipeline reactor. One disadvantage to the pipeline process is that it requires long residence times that can lead to product degradation.
U.S. Pat. No. 6,201,096 discloses a process for the production of a polyamide by reaction of an omega-aminonitrile with water in a vertical multistage reactor that is swept by steam. For example, this process can convert 6-aminocapronitrile to nylon-6. Complete hydrolysis of the large number of nitrile ends requires residence times of 2 to 4 hours.
U.S Pat. No. 2,357,484 discloses a catalyzed vapor phase process for high conversion and rapid hydrolysis of the nitrile group in 6-aminocapronitrile to produce caprolactam. Because of the residual 6-aminocapronitrile that remains in the caprolactam product, it must be purified before using in a conventional polymerization.
It would be desirable to have a process for the conversion of 6-aminocapronitrile to nylon-6 that does not require long liquid residence times that can lead to product degradation and does not require additional purification steps.
The present invention is a process for the production of nylon 6 which comprises:
a) (1) contacting either steam or liquid water and 6-aminocapronitrile in the presence of a dehydration catalyst to produce a first feed stream comprising a major portion of caprolactam and a minor portion of 6-aminocapronitrile, or (2) contacting 6-aminocapronitrile and water at elevated temperature and superatmospheric pressure to produce a second feed stream comprising a major portion of nylon 6 prepolymer having more than about 40 gram equivalents of untreated nitrile end groups per million grams of nylon 6 prepolymer, and a minor portion of 6-aminocapronitrile;
b) providing a vertical countercurrent multistage reactor having a top and a bottom, and having upper stages and lower stages, said reactor being equipped with internal perforated barrier means for establishing a plurality of stages and for effecting contact of said first and/or second feed stream with a countercurrently-flowing steam stream;
c) introducing at least one of said first and second feed streams directly into said reactor at or near the top of said reactor;
d) introducing steam into said reactor at at least one introduction point near the bottom of said reactor;
e) maintaining a pressure within said reactor between 50 and 800 psig (0.34 and 5.52 MPa) and a temperature at the top of said reactor between 190 and 250 degrees C. and a temperature at the bottom of said reactor between 260 and 290 degrees C.;
f) withdrawing a steam-containing overhead stream at the top of said reactor; and
g) recovering a product stream from the bottom of said reactor, said product stream comprising nylon 6 polymer having less than about 20 gram equivalents of unreacted nitrile end groups per million grams of nylon 6.
By introducing directly into the vertical multistage reactive distillation column the product of the reaction step that produces the feed containing mostly caprolactam (first embodiment), a process is provided that should not need an additional costly purification step of an intermediate product.
By introducing directly into the vertical multistage reactive distillation column the product of the reaction step that produces the feed containing mostly nylon 6 prepolymer (second embodiment), a process is provided that should not need excessively long liquid residence times that can lead to degradation reactions.